not just once but half a dozen times and senator dan quayle in 1978. >> i knew jab jack kennedy. jack kennedy was a friend of mine. senator, you're no jack kennedy. >> reporter: memorable moments but hardly game-changers. vice-presidential debates like this week's upcoming matchup between vice president biden and his gop challenger paul ryan are a different animal. >> you need to be able to see that each vice-presidential candidate has hit the standard of readiness, that they'll be ready in the event of a national crisis to step into the oval office and take the reins of presidency. beyond that, unless there's a star-killing or star-catching moment, it rarely changes the course of the debate. >> reporter: democratic strategist donna hill says the first step here is to do no harm. >> try not to break any ground. this is not an audition for 2016. >> i don't like a damn thing about it. >> reporter: biden who only participated in a few de-bates says the contests are never easy. >> what i've been doing mostly is, quite frankly, studying up on congress ryan's positions on the issues, a